Overcoming Nanoscale Inhomogeneities in Thin-Film Perovskites via Exceptional Post-annealing Grain Growth for Enhanced Photodetection

Antisolvent-assisted spin coating has been widely used for fabricating metal halide perovskite films with smooth and compact morphology. However, localized nanoscale inhomogeneities exist in these films owing to rapid crystallization, undermining their overall optoelectronic performance. Here, we show that by relaxing the requirement for film smoothness, outstanding film quality can be obtained simply through a post-annealing grain growth process without passivation agents. The morphological changes, driven by a vaporized methylammonium chloride (MACl)–dimethylformamide (DMF) solution, lead to comprehensive defect elimination. Our nanoscale characterization visualizes the local defective clusters in the as-deposited film and their elimination following treatment, which couples with the observation of emissive grain boundaries and excellent inter- and intragrain optoelectronic uniformity in the polycrystalline film. Overcoming these performance-limiting inhomogeneities results in the enhancement of the photoresponse to low-light (<0.1 mW cm–2) illumination by up to 40-fold, yielding high-performance photodiodes with superior low-light detection.


Deposition of perovskite film
The precursor solution of CH3NH3PbI3 (MAPbI3) perovskite was prepared by codissolving equal molar ratio of lead iodide (PbI2, 99.985%) and methylammonium iodide (MAI) in a mixed solvent of N,N-Dimethylmethanamide (DMF) and Dimethyl sulfoxide (DMSO) (9:1.1 in volume). The concentration of PbI2 and MAI were both 1.5 M. The solution was stirred at 60°C until the precursors were fully dissolved. 40 μl precursor solution was dropped onto substrates with or without hole transport layers and was spun at 4000 rpm for 30s. At the seventh second, 0.5 ml diethyl ether was dripped onto the spinning substrate. The MAPbI3 reference films were annealed on a hot plate at 100°C for 15 minutes. For MACl treatment the films were annealed for 5 minutes before transferring to a cylindrical chamber.

MACl treatment
The setup is modified from an aerosol assisted chemical vapor deposition system. Approximately 4 ml of MACl solution in DMF was aerosolized inside a bubbler by a piezoelectric ultrasonic generator. This was carried to a cylindrical chamber by nitrogen flow at 0.5 L min -1 through a planarizing baffle. The aerosol forms a laminar flow, steadily passes over the film and is drained on the other side of chamber. The substrate was heated at 100°C by a graphite block.

Structural and morphological characterization
X-ray diffraction (XRD) patterns of perovskite films were obtained with an X'pert Powder diffractometer (PANalytical), Cu K source. The diffraction patterns were measured over the range 7 -40° 2, with the sample rotating during measurement. The top-view, tilt-angle and cross-sectional scanning electron microscopy (SEM) images were obtained using a LEO Gemini 1525 field emission gun scanning electron microscopy. The working voltage of SEM was fixed at 5 kV. To prevent charging, all the films were coated with a 10 nm chromium layer.

Spectroscopic characterization
Steady-state photoluminescence (PL) spectra were measured with a commercial FL 1039 spectrometer (Horiba Scientific), measured with 635-nm monochromatic laser excitation at intensity of 1.5 mW cm -2 . PL decay was measured with a time-correlated single photon counting (TCSPC) spectrometer on a Delta Flex system (Horiba Scientific). The excitation was provided by a 435-nm monochromatic laser diode. The excitation density was measured by a power meter and was tuned by inserting neutral density optical filters.

Hyperspectral fluorescence mapping.
Hyperspectral photoluminescence maps were performed using an IMA TM Vis widefield optical microscope from Photon Etc. Samples were illuminated using a 405 nm continuous wave laser at an intensity of ~240 mW cm -2 through an Olympus 100× objective. A 415 nm long pass filter removed the laser emission from the collected light.
The incident angle of the emitted photoluminescence onto a volume Bragg grating was varied to spectrally split the light incident onto the Hamamatsu CMOS camera.
Reconstruction of the data allowed the extraction of photoluminescence spectra at each point on the sample.

Photoconductive AFM measurement
Photoconductive atomic force microscopy (pc-AFM) measurements were performed on an AIST-NT Combiscope 1000 AFM system. The AFM system was enclosed in a glovebox (Jacomex) under dry nitrogen conditions (~10 ppm O2; 1 ppm H2O). Aucoated 240AC-GG Opus probes from MicroMasch, with nominal resonance frequency at 70 kHz and 2 N/m force constant, were used for all experiments. Assuming nominal force constants, contact mode setpoint forces during pc-AFM were kept between 2 nN and 3 nN and no tip induced damage on the film surface was observed throughout measurements. The tip-at-the-front geometry of the selected probe model enabled optical excitation without tip-shadowing.
Optical excitation was coupled in laterally using a 633 nm He-Ne laser beam. The lateral coupling angle was fixed at approximately 30° from the sample plane. The incident optical power was adjusted by placing a neutral density filter wheel in the beam path.

Fabrication and characterization of perovskite photodiode (PPD)
The PPDs were fabricated on indium-doped tin oxide (ITO) coated glass substrates.
Current density-voltage (J-V) characteristics of the PPDs were measured using a Keithley 2400 source meter, in the dark or illuminated by an AM 1.5 filtered xenon lamp (Oriel Instruments) at 1 sun intensity (100 mW cm -2 ), calibrated using a silicon reference photodiode. The scan rate was fixed at 10 mV s -1 in both forward directions (-2 V to 2 V).
For determination of the Linear Dynamic Range (LDR), the light was attenuated using a selection of neutral density filters placed between the lamp and PPD. The photocurrent (Jph) was calculated as the difference in response between the illuminated current density (Jlight) and dark current density (Jd) at each light intensity. Responsivity was measured using an integrated system from Quantum Design PV300. All the devices were tested in ambient air. Figure S1. Surface SEM images of Perovskite films treated by DMF aerosol with different concentration of MACl.

Solid-state crystal growth
Solid-state crystal growth can be achieved by maintaining the heating temperature of MAPbI3 films at or above 100°C. Decreasing the heating temperature results in DMF completely "wetting" the film, and a discontinuous morphology is observed when the film is re-dried. These data show that it is only possible to obtain grain growth whilst maintaining compact film morphology when the film remains in the solid state.         Table S1.